In gas turbine engines, the low pressure or booster compressor rotor(s) are typically driven by the low pressure spool either by direct connection thereto such that they rotate at a same rotational speed, or through a fixed ratio gearbox. However, the speed of the low pressure spool is usually determined by the load requirements of the engine, whether the load includes a fan, an output shaft, a propeller, or any other adequate type of rotatable load. In particular for turboprop, turboshaft or APU engines, the rotatable load may be required to rotate at a constant or approximately constant rotational speed throughout a range of power demands.
In addition, because power demands on the engine vary, for example between take-off and cruise conditions, the turbine and compressor rotors of the core section typically have to rotate at a relatively large range of rotational speeds. For example, low power requirement conditions may require the rotors of the core section to rotate relatively far below their optimal rotational speed. This in turn may affect the rotational speed of the low pressure turbine rotor(s), and as such of the low pressure spool and associated low pressure compressor rotor(s), which may limit the engine's efficiency in such conditions.